Liquid-semiconductor photocell using sintered electrode

ABSTRACT

Liquid-semiconductor photocells have received attention recently for use in solar power devices. Alternatives to single crystal semiconductors have been sought to reduce the cost of the photocells. According to this invention, the semiconductor is made from a pressure sintered and vapor annealed semiconductor. The electrode is relatively inexpensive to make and the efficiency of the solar cell compares favorably to the efficiency of solar cells using single crystal electrodes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to large area semiconductor junctiondevices for use as photocells and in particular to such devices for useas solar cells.

2. Description of the Prior Art

Concern over the continued availability of fossil fuel energy sourceshas generated interest in the development of other energy sourcesincluding solar power which can be used to generate electricity. Thedevices most often considered for conversion of solar power intoelectricity are semiconductor devices, commonly called solar cells,which collect light, and generate photocurrent, in proportion to thearea of the photosensitive junction which must be large to generate auseful current. The cost of manufacturing such devices depends mainly onthe area of the photosensitive junction and is presently too high topermit commercial exploitation of solar cells for other than limited andspecialized applications.

Considerable effort has been devoted to finding ways to reduce the costof semiconductor solar cell devices. Much of this effort has beendirected, as in U.S. Pat. No. 3,953,876 issued Apr. 27, 1976, to devicesin which the semiconductor material is deposited as a polycrystallinethin film on an inexpensive substrate rather than grown by the costlysingle crystal techniques used in early solar cells. A differentapproach that has generated enthusiasm recently is the liquidsemiconductor junction solar cells. The active part of these cells is ajunction formed at a semiconductor-liquid interface. Because thejunction forms spontaneously at the liquid-solid interface, the devicepromises to be less costly to manufacture as relatively costly epitaxyor diffusion procedures, required for the single crystal orpolycrystalline devices mentioned, are not needed to form the junction.

Two obstacles still remain and must be surmounted before such cells canbe exploited commercially. First, liquid-semiconductor junctions areoften not photochemically stable because photoexcitation produces holesat the semiconductor surface which may react with the redox electrolyteand corrode the semiconductor surface in a manner that degrades thedesired characteristics of the semiconductor surface as manifested bydecay of the photocurrent from the cell with operating time. An exampleof such a reaction with a CdS electrode is CdS+2h⁺ →S⁰ +Cd²⁺ leading tothe formation of a sulfur layer at the junction interface. One approachto this problem involves the use of a polysulfide-sulfide redox couplesolution. Since the corrosion reaction CdS+2h⁺ →Cd⁺⁺ +S proceeds at ahigher electrode potential than the reaction S⁼ →S+2e, thesulfur-polysulfide couple consumes the holes responsible for thecorrosion reaction before the potential for the corrosion reaction isreached.

Second, the cost of single crystal semiconductor electrodes is too highfor commercial success. Several approaches have been tried to reduce thecost of the single crystal semiconductor, especially chalcogenide,electrode. One involves the electrolytic codeposition of the electrodematerials, e.g., cadmium and selenium, on an inert substrate. Anotherinvolves the anodization of a cadmium or bismuth substrate to form achalcogenide semiconductor.

SUMMARY OF THE INVENTION

We have discovered that the semiconductor element for aliquid-semiconductor junction photocell can be produced by sinteringpowdered semiconductor material under controlled temperature andpressure and then vapor annealing the sintered material at controlledtemperature. Chalcogenide semiconductors appear especially suited to theinvention and a CdSe electrode produced in this way is both inexpensiveto produce and relatively efficient as compared to single crystal CdSeelectrodes.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a plot of the theoretical energy conversion efficiency forseveral semiconductor materials as a function of bandgap taking thesolar spectrum into account;

FIG. 2 is a schematic representation of a liquid-semiconductorphotocell; and

FIG. 3 is a plot of photocurrent vs. voltage for a CdSe liquid photocellmade according to this invention.

DETAILED DESCRIPTION

FIG. 1 shows an idealized plot of energy conversion efficiency forseveral semiconductor materials versus semiconductor bandgap taking thesolar spectrum into account. The range of efficiencies for each bandgapvalue results from different atmospheric conditions and assumptionsabout losses in cell voltage. As can be seen, CdSe, CdTe, CdS and Bi₂ S₃have bandgaps that permit approximately the maximum energy conversionefficiency theoretically possible.

The cell structure of FIG. 2 comprises a container 20, electrolyte 21,counter electrode 22, which in our devices is carbon, although otherinert materials may be used, and the active electrode 23. Electrode 23is insulated as with epoxy 24 except where activated and illuminated.The container may be made of any conveniently available glass or plasticmaterial. An aqueous electrolyte is preferred because of the betterconductivity it affords although nonaqueous electrolytes may also beused. The bottom of the cell is transparent to pass incident light asshown. Photocells as just described were made with various sintered andvapor-annealed semiconductors as the active electrode 24.

Semiconductor powder of high purity, e.g., typically of 99.999% orgreater purity and having particle sizes ranging from 1 to 100 micronsis sintered at a temperature in the range from 600° to 1100° C under apressure in the range from 4000 psi to 10,000 psi. The resulting disksare sliced and vapor annealed in evacuated quartz tubes over metalvapor, for 1 to 120 hours and with a temperature between 500° and 800°C, until stoichiometry is restored and the desired carrier concentrationis reached. The preferred dopant for the Cd chalcogenides is Cd and forthe Bi chalcogenides is Bi. The desired dopant concentration is lessthan 5 × 10¹⁸ /cm³ because above this value the space charge layer istoo thin to permit light absorption only within the space charge layer.Electrical contacts such as indium and silver epoxy are then made to thedisks with conventional techniques.

The above temperature and pressure ranges have been found to be not verycritical. The temperature and pressure ranges mentioned are sufficientlyhigh to cause the growth of grains larger than the 1 microns grainsdesired. Upon heating to temperatures necessary to achieve grain growth,the material does, however, lose its stoichiometry through the loss ofmaterial, for example, from a chalcogenide, Cd and is not suitable forelectrode use at this time because it is highly resistive and possessesthe wrong doping level. It is not known with any precision why thematerial possesses these undesirable properties prior to metal vaporannealing. Possible causes may be crystal imperfections associated withthe lack of stoichiometry or phase transitions caused by the hightemperatures and pressures used. The annealing step restores the properamount of material, e.g., Cd or Bi and makes the material a properlydoped n-type semiconductor. If they are previously present, theannealing also reduces the number of crystal imperfections and restoresthe material to the desired phase. As such, annealing is a critical stepand is carried out in the presence of metal vapor at a temperaturebetween 500° and 700° C for a time period between 1 hour and 140 hours.Within these ranges the intervals between 550° and 600° C and between 10hours and 100 hours have been found to give best results for Cd vaporannealing.

High efficiencies will be obtained in solar cells using polycrystallinematerials only if the grain size is sufficiently large to absorbpractically all incident light in the top layer of grains exposed to theelectrolyte. Additionally, all efficient photovoltaic devices requirethat the space charge layer thickness should be less than the lightabsorption depth, and traps due to lattice mismatch or dislocations ator near the absorbing junction must be eliminated or minimized innumber.

The absorption length must be less than the grain size because lightabsorbed beyond the first layer of grains does not effectively add tothe photocurrent as minority carriers are efficiently trapped at thegrain boundaries. The absorption lengths for contemplated materialsincluding CdSe, CdTe, CdS, and Bi₂ S₃ are approximately 10⁻⁴ -10⁻⁵ cmand grain sizes of 1μ are adequate. The large size of the grainsproduced by the sintering, typically 10μ or larger, compared to theabsorption length accounts for the relatively great latitude allowed forthe temperatures and pressures used in the sintering process.

The thickness of the space charge layer must be less than the absorptionlength to insure prompt separation of the carriers and reduce theprobability of their recombination. As is known, the thickness, S, ofthe space charge layer is given by ##EQU1## where ε is the staticdielectric constant of the semiconductor material, ε_(o) is thepermittivity of free space, Δψ_(sc) is the voltage drop across the spacecharge layer, q is the electron charge and N_(D) is the donorconcentration.

Traps due to lattice mismatch or dislocations at or near the junctionare undesirable because they cause recombination of carriers whichreduces cell efficiency. The problem of lattice mismatch is inherentlylacking at liquid-solid interfaces and if the grains are virtual singlecrystals, etching can remove surface defects at the upper layer. It isdesirable that the electrode be etched as in a 3:1 to 4:1 mixture of HCland HNO₃, to remove surface defects. It has been found thatpolysulfide-sulfide, polytelluride-telluride, and polyselenide-selenideredox electrolytes permit operation of the cells over extended timeperiods with minimal photocorrosion of the electrode. The maximumelectrolyte concentration is determined by the maximum amount that theymay be dissolved in the solute. The minimum concentration is determinedby the need of the electrolyte to carry a useful amount of photocurrentand still prevent excessive photoetching and is approximately 0.1 molarfor the mentioned redox couples in an aqueous solution.

The current-voltage characteristic curve for a cell with apolysulfide-sulfide redox electrolyte, a nominal 1 molar total sulfideconcentration and a CdSe electrode is shown in FIG. 3. The illuminationused was sunlight equivalent to noon time winter illumination on middlelatitudes--air mass two (AM2). The efficiency of the cell is 5.1% orabout 68% of the value obtained with a single crystal electrode.

EXAMPLES

CdTe powder of 99.999% purity and 5-10 μm particle size was pressed at650° C and at 10,000 psi for two hours. The resulting pellet wascomposed of grains between 20 μm and 30 μm diameter. The pellet wasannealed in a sealed tube containing Cd vapor at 600° C for 100 hours.The resulting pellet was used in a photocell having a nominal 1 molartotal selenium concentration. H₂ Se was dissolved in a basic solutionsuch as KOH to obtain the electrolyte. Other bases could be used. Theshort circuit current density of this cell, under illumination by a 100watt tungsten halogen lamp was 13.3 ma/cm² and its open circuit voltagewas 0.77 volts. A solar cell made with a single n-type CdTe crystal, inthe same solution and under similar illumination, had a short circuitcurrent density of 45.8 ma/cm² and an open circuit voltage of 0.76volts.

CdSe electrodes were prepared from CdSe material of greater than 99.999%purity and 5-10 μm particle size as shown in Table 1. The cells were rununder a light flux approximating AM2 conditions with approximately a 1mole sulfide/polysulfide redox electrolyte and a carboncounterelectrode. The short circuit current and conversion efficiencyare given with respect to values obtained with a single CdSe crystal.

                                      Table 1                                     __________________________________________________________________________     Method of    Method of                                                                              Grain         Relative                                                                             Open                              Preparation   Cd Anneal                                                                              Size,                                                                              Relative Short                                                                         Conversion                                                                           Circuit                                                                             Fill                        T° C                                                                        P,kpsi                                                                             t,hrs                                                                             T° C                                                                        t,hrs                                                                             Micron                                                                             Circuit Current                                                                        Efficiency                                                                           Voltage                                                                             Factor                      __________________________________________________________________________    925  10   2   600  16  10-20                                                                              0.70     0.69   0.730 0.58                        940  10   2   600  16  10-20                                                                              0.68     0.60   0.728 0.56                        1100  4   1   600  17  10-20                                                                              0.55     0.48   0.710 0.52                        940  10   2   700  112 10-20                                                                              0.68     0.70   0.755 0.49                        __________________________________________________________________________

What is claimed is:
 1. A photocell containing a photovoltaic junctionbeteen a semiconductor material and a liquid electrolyte containing aredox couple CHARACTERIZED IN THAT said photoactive electrode comprisesa sintered and metal vapor annealed semiconductor, said semiconductorbeing formed from a powdered chalcogenide selected from the groupconsisting of cadmium selenide, cadmium telluride, cadmium sulfide andbismuth sulfide and mixtures thereof, and said metal is selected fromthe group consisting of cadmium and bismuth.
 2. The photocell recited inclaim 1 in which said electrolyte containing the redox couple is asolution comprising anions selected from the group consisting ofsulfide, selenide, telluride and mixtures thereof.
 3. A method formaking a photocell comprising:forming a chalcogenide semiconductorelectrode, said chalcogenide being selected from the group consisting ofpowdered cadmium selenide, cadmium sulfide, cadmium telluride andbismuth sulfide; and immersing said electrode and a counter electrode ina cell containing a redox electrolyte comprising anions selected fromthe group consisting of sulfide, selenide, telluride and mixturesthereof; CHARACTERIZED IN THAT said forming step comprises: sinteringand metal vapor annealing said chalcogenide semiconductor, said metalbeing selected from the group consisting of cadmium and bismuth.
 4. Amethod as recited in claim 3 in which said sintering step uses apressure between 400psi and 10,000psi and a temperature between 600° and1100° C.
 5. A method as recited in claim 4 in which said metal vaporannealing step comprises heating said chalcogenide to a temperaturebetween 500° and 800° C and exposing said chalcogenide to said metalvapor.